xfs/31[2-7]: hide pwrite fsync errors that are supposed to fail

xfs/31[2-7]: hide pwrite fsync errors that are supposed to fail

xfs_io 4.14 will gain the ability to print error messages when
pwrite+fsync fail.  Certain tests use the error injector to cause
failures, so the errors are expected.  Since we test for a shut down
filesystem after the error injection, we can push the error messages to
the log.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>